The amino acid nature of ampicillin and related penicillins

pKa Values of Medicinal Compounds in Pharmacy Practice

A brief review of pKa values and their uses in pharmacy practice is presented. Tables of pKa values for 400 medicinal compounds, of pH values of 18 body fluid sites, and of electron-inducing chemical substituents are provided. Adequate literature sources are included to permit further reference to the more detailed aspects of conditions associated with pKa value determinations or uses.

Label-free microfluidic free-flow isoelectric focusing, pH gradient sensing and near real-time isoelectric point determination of biomolecules and blood plasma fractions

Continuous biomolecular separation and pH gradient observation using UV and NIR fluorescence.

Mechanism of chemical degradation and determination of solubility by kinetic modeling of the highly unstable sesquiterpene lactone nobilin in different media

The objective of this work was first to investigate the chemical degradation of the sesquiterpene lactone nobilin and determine its solubility under conditions of concurrent degradation for partially amorphous starting material; second, to determine the effect of biorelevant media used in the in vitro measurement of intestinal absorption on degradation and solubility of nobilin. Purely aqueous medium (aq-TMCaco ), fasted and fed state simulated intestinal fluid (FaSSIF-TMCaco and FeSSIF-TMCaco ), and two liposomal formulations (LiposomesFaSSIF and LiposomesFeSSIF ) with the same lipid concentration as FaSSIF-TMCaco and FeSSIF-TMCaco were used. Degradation products were identified by nuclear magnetic resonance and X-ray crystallography and the order of reaction kinetics was determined. Solubility was deduced with a mathematical model encompassing dissolution, degradation, and reprecipitation kinetics that took into account particle size distribution of the solid material. Degradation mechanism of nobilin involved water-catalyzed opening of the lactone ring and transannular cyclization resulting in five degradation products. Degradation followed first-order kinetics in aq-TMCaco and FaSSIF-TMCaco , and higher-order kinetics in FeSSIF-TMCaco and the two liposomal formulations, whereas degradation in the latter media was diminished. Solubility of nobilin increased in the order: aq-TMCaco < FaSSIF-TMCaco , < LiposomesFaSSIF < FeSSIF-TMCaco < LiposomesFeSSIF . Improvement of stability and solubility was consistent with the incorporation of the nobilin molecule into colloidal lipid particles. The developed kinetic model is proposed to be a useful tool for deducing solubility under dynamic conditions.

Modeling solubility, acid-base properties and activity coefficients of amoxicillin, ampicillin and (+)6-aminopenicillanic acid, in NaCl(aq) at different ionic strengths and temperatures

The total solubility of three penicillin derivatives was determined, in pure water and NaCl aqueous solutions at different salt concentrations (from ∼0.15 to 1.0 mol L(-1) for ampicillin and amoxicillin, and from ∼0.05 to 2.0 mol L(-1) for (+)6-aminopenicillanic acid), using the shake-flask method for generating the saturated solutions, followed by potentiometric analysis. The knowledge of the pH of solubilization and of the protonation constants determined in the same experimental conditions, allowed us to calculate, by means of the mass balance equations, the solubility of the neutral species at different ionic strength values, to model its dependence on the salt concentration and to determine the corresponding values at infinite dilution. The salting parameter and the activity coefficients of the neutral species were calculated by the Setschenow equation. The protonation constants of ampicillin and amoxicillin, determined at different temperatures (from T=288.15 to 318.15K), from potentiometric and spectrophotometric measurements, were used to calculate, by means of the Van't Hoff equation, the temperature coefficients at different ionic strength values and the corresponding protonation entropies. The protonation enthalpies of the (+)6-aminopenicillanic acid were determined by isoperibol calorimetric titrations at T=298.15K and up to I=2.0 mol L(-1). The dependence of the protonation constants on ionic strength was modeled by means of the Debye-Hückel and SIT (Specific ion Interaction Theory) approaches, and the specific interaction parameters of the ionic species were determined. The hydrolysis of the β-lactam ring was studied by spectrophotometric and H NMR investigations as a function of pH, ionic strength and time. Potentiometric measurements carried out on the hydrolyzed (+)6-aminopenicillanic acid allowed us to highlight that the opened and the closed β-lactam forms of the (+)6-aminopenicillanic acid have quite different acid-base properties. An analysis of literature solubility, protonation constants, enthalpies and activity coefficients is reported too.

Enzymatic synthesis of ampicillin: nonlinear modeling, kinetics estimation, and adaptive control

Nowadays, the use of advanced control strategies in biotechnology is quite low. A main reason is the lack of quality of the data, and the fact that more sophisticated control strategies must be based on a model of the dynamics of bioprocesses. The nonlinearity of the bioprocesses and the absence of cheap and reliable instrumentation require an enhanced modeling effort and identification strategies for the kinetics. The present work approaches modeling and control strategies for the enzymatic synthesis of ampicillin that is carried out inside a fed-batch bioreactor. First, a nonlinear dynamical model of this bioprocess is obtained by using a novel modeling procedure for biotechnology: the bond graph methodology. Second, a high gain observer is designed for the estimation of the imprecisely known kinetics of the synthesis process. Third, by combining an exact linearizing control law with the on-line estimation kinetics algorithm, a nonlinear adaptive control law is designed. The case study discussed shows that a nonlinear feedback control strategy applied to the ampicillin synthesis bioprocess can cope with disturbances, noisy measurements, and parametric uncertainties. Numerical simulations performed with MATLAB environment are included in order to test the behavior and the performances of the proposed estimation and control strategies.

In situ artificial membrane permeation assay under hydrodynamic control: correlation between drug in vitro permeability and fraction absorbed in humans

The purpose of this study was to develop an in vitro permeation model that will predict the fraction of drugs absorbed in humans. A rotating-diffusion cell with two aqueous compartments, separated by a lipid-impregnated artificial membrane, was used to determine the permeability of drugs under conditions of controlled hydrodynamics. The measured effective permeability coefficient was modified to include the paracellular transport derived from a previously reported colorectal adenocarcinoma epithelial cell line (Caco-2) permeability study and the effects of unstirred water layer anticipated in vivo. Permeability data were collected for 31 different marketed drugs with known absolute oral bioavailability and human hepatic clearance data. Literature bioavailability values were corrected for the first pass hepatic clearance thus obtaining the fraction absorbed from intestinal lumen (fraction absorbed), F(a), while assuming that the fraction escaping intestinal extraction, F(g), was approximately ~1. Permeability obtained under conditions of controlled hydrodynamics was compared with the permeability measured under unstirred conditions. It is shown that the optimized effective permeability correlates with the fraction absorbed. In contrast, permeability data obtained under unstirred conditions does not show a good correlation. The in vitro permeation model developed in this study predicts the fraction absorbed of the selected drugs in humans within experimental uncertainty. It has been demonstrated that the correlation with the fraction absorbed is greatly improved using the permeability data obtained under controlled hydrodynamics with paracellular transport included in the model.

The influence of potassium clavulanate on the rate of amoxicillin sodium degradation in phosphate and acetate buffers in the liquid state

The stability of aqueous admixtures of amoxicillin sodium and potassium clavulanate was studied in the liquid state at selected pH values. Potassium clavulanate was found to catalyze the rate of degradation of amoxicillin sodium under the conditions of this study. In phosphate buffer (at pH 7.0) both amoxicillin sodium and potassium clavulanate showed first-order degradation when stored separately. However, when combined the rate of amoxicillin degradation increased and t(90) values for amoxicillin decreased from 69.6 min for amoxicillin alone to 10.8 min for amoxicillin in the combination at 55 degrees C. A kinetic model was developed that explained the catalytic behavior of potassium clavulanate and phosphate buffer. In acetate buffer the rate of degradation of amoxicillin sodium followed first-order kinetics, but the catalytic effect of clavulanate caused curvature in the rate plots at higher temperatures and clavulanate concentrations. This catalytic effect was less than that occurred in phosphate buffer (where the t(90) value of amoxicillin decreased from 137.3 min for amoxicillin alone to 52.5 min for amoxicillin in combination at 55 degrees C). First-order bi-exponential decay occurred with amoxicillin degradation, which explained this change in rate.

Interaction of zwitterionic penicillins with the OmpF channel facilitates their translocation

To study translocation of beta-lactam antibiotics of different size and charge across the outer bacterial membrane, we combine an analysis of ion currents through single trimeric outer membrane protein F (OmpF) porins in planar lipid bilayers with molecular dynamics simulations. Because the size of penicillin molecules is close to the size of the narrowest part of the OmpF pore, penicillins occlude the pore during their translocation. Favorably interacting penicillins cause time-resolvable transient blockages of the small-ion current through the channel and thereby provide information about their dynamics within the pore. Analyzing these random fluctuations, we find that ampicillin and amoxicillin have a relatively high affinity for OmpF. In contrast, no or only a weak interaction is detected for carbenicillin, azlocillin, and piperacillin. Molecular dynamics simulations suggest a possible pathway of these drugs through the OmpF channel and rationalize our experimental findings. For zwitterionic ampicillin and amoxicillin, we identify a region of binding sites near the narrowest part of the channel pore. Interactions with these sites partially compensate for the entropic cost of drug confinement by the channel. Whereas azlocillin and piperacillin are clearly too big to pass through the channel constriction, dianionic carbenicillin does not find an efficient binding region in the constriction zone. Carbenicillin's favorable interactions are limited to the extracellular vestibule. These observations confirm our earlier suggestion that a set of high-affinity sites at the narrowest part of the OmpF channel improves a drug's ability to cross the membrane via the pore.

Designed to penetrate: time-resolved interaction of single antibiotic molecules with bacterial pores

Membrane permeability barriers are among the factors contributing to the intrinsic resistance of bacteria to antibiotics. We have been able to resolve single ampicillin molecules moving through a channel of the general bacterial porin, OmpF (outer membrane protein F), believed to be the principal pathway for the beta-lactam antibiotics. With ion channel reconstitution and high-resolution conductance recording, we find that ampicillin and several other efficient penicillins and cephalosporins strongly interact with the residues of the constriction zone of the OmpF channel. Therefore, we hypothesize that, in analogy to substrate-specific channels that evolved to bind certain metabolite molecules, antibiotics have "evolved" to be channel-specific. Molecular modeling suggests that the charge distribution of the ampicillin molecule complements the charge distribution at the narrowest part of the bacterial porin. Interaction of these charges creates a region of attraction inside the channel that facilitates drug translocation through the constriction zone and results in higher permeability rates.

Stabilization of eptifibatide by cosolvents

Eptifibatide is a potent and highly specific inhibitor of platelet receptor glycoprotein IIb/IIIa and is indicated in the treatment of acute coronary syndrome. The commercial product Integrilin(R) (eptifibatide) Injection requires a cold/refrigerator storage condition. In an effort to improve the drug stability for room temperature storage and transportation, this study proposed a semi-aqueous formulation that contains 2 mg/ml eptifibatide, 10% ethanol, 40% propylene glycol and 50% 0.025 M citrate buffer. The stability study was conducted in the pH range 4.25-6.25 under accelerated temperatures: 48, 60, 72.5 degrees C. The results indicate that the proposed semi-aqueous vehicles substantially increased the drug stability in comparison with aqueous vehicles. The predicted drug shelf-life T(90) at 25 degrees C shows that an almost twofold increase can be achieved by formulating eptifibatide in the semi-aqueous vehicle, which is 60 months at its maximum stability of pH 5.75, as opposed to the 33 months in the aqueous vehicle at its maximum stability of pH 5.25.

Enthalpies of solution of ampicillin, amoxycillin and their binary mixtures at 310.15 K

Enthalpies of solutions of ampicillin, amoxycillin and their binary mixtures have been determined at pH 2, 5, and 7 using C-80 calorimeter. The systems showed endothermic behaviour; molar enthalpies of solutions of ampicillin were determined to be 13.32, 15.89 and 23.21 kJ mol(-1) and amoxycillin were 16.32, 18.45 and 26. 25 kJ mol(-1) at pH 2, 5, and 7, respectively. The excess molar enthalpies of solution have also been calculated to find any interaction between these two drugs.

Transport studies of beta-lactam antibiotics and their degradation products across electrified water/oil interface

An electrochemical method for quantifying beta-lactam antibiotics (cephalexin and ampicillin) and their hydrolysis products is described. Cyclic voltammetry at the water/nitrobenzene interface in a four-electrode system was used. The zwitterionic compounds were ionized to the necessary electrochemical form by pH adjustment. The pH change, however, resulted also in hydrolysis of the antibiotics. Hydrolysis products were characterized across UV-vis spectrum. The various hydrolysis products as well as the ionized antibiotics were studied in voltammetric transfer from water to nitrobenzene using the method of the interface between two immiscible electrolyte solutions (ITIES). It was concluded that this electrochemical method is suitable for the quantification of beta-lactam antibiotics and their hydrolysis products.

Polarographic determination of some penicillins through nitrosation

Direct current and differential pulse polarography DPP were used for the determination of three penicillins, namely, ampicillin, benzylpenicillin and carbenicillin, in pure form and in their dosage forms. The method is based upon treatment of penicillins with nitrous acid followed by polarographic measurement of the produced derivatives polarographically. The nitroso derivatives formed exhibited reduction waves over the whole pH range in Britton-Robinson buffers. The waves were characterized as being diffusion-controlled and free from adsorption phenomena. The current-concentration plots were rectilinear over the concentration range 8-200 and 2-160 micrograms ml-1 for DCt and DPP, respectively, for all the studied compounds. The proposed method was further applied to determine penicillins in pharmaceutical preparations, and the results obtained were in good agreement with those given by the companies.

Stability of penicillin G, ampicillin, amikacin and oxytetracycline and their interactions with food in in vitro simulated equine gastrointestinal contents

Penicillin G was extensively (84.7 per cent) and amikacin moderately (14.4 per cent) degraded when incubated for one hour in a chloride buffer at pH 1.9 designed to mimic the equine gastric pH. Ampicillin and oxytetracycline were stable at pH 1.9. Penicillin and ampicillin were moderately stable (more than 90 per cent) when incubated in equine caecal liquor for three hours but were degraded by about 65 per cent after 24 hours. More than 80 per cent of the initial concentrations of amikacin and oxytetracycline were recovered after 24 hours' incubation in equine caecal liquor. The concentrations of short chain fatty acids in equine caecal liquor were not affected by incubation with penicillin G, ampicillin, amikacin or oxytetracycline. More than 84 per cent of penicillin G and amikacin became bound to hay in buffers at pH 1.9 and pH 7.0. Ampicillin did not become bound to hay at pH 1.9, but more than 60 per cent became bound at pH 7.0.

Thermodynamics of the hydrolysis of penicillin G and ampicillin

Apparent equilibrium constants and calorimetric enthalpies of reaction have been measured for the beta-lactamase catalyzed hydrolysis of penicillin G(aq) and ampicillin(aq) to penicillinoic acid(aq) and to ampicillinoic acid(aq), respectively. High-pressure liquid-chromatography and microcalorimetry were used to perform these measurements. The results for the reference reactions at T = 298.15 K and Im = 0 are: Ko = (9.4 +/- 3.1) x 10(-7), delta rGo = (34.4 +/- 1.0)kJ mol-1, delta rHo = -(73.7 +/- 0.4)kJ mol-1, and delta rSo = -(363 +/- 4) J K-1 mol-1 for penicillin G-(aq) + H2O(1) = penicillinoic acid2-(aq) + H+(aq); Ko = (6.0 +/- 3.0) x 10(-6), delta rGo = (29.8 +/- 1.7) kJ mol-1, delta rHo = -(70.0 +/- 7.5) kJ mol-1, and delta rSo = -(335 +/- 26) J K-1 mol-1 for ampicillin-(aq)+ H2O(1) = ampicillinoic acid2-(aq)+H+(aq). Calorimetric enthalpies of reaction for the beta-lactamase catalyzed hydrolysis of cephalosporin C have also been measured but the reaction products have not been identified and the measured enthalpies cannot be assigned to a specific reaction. Acidity constants for ampicillin, penicillin G, ampicillinoic acid, and penicillinoic acid are also reported. A strain energy of 116 kJ mol-1 for the beta-lactam ring is obtained from thermochemical data.

Ampicillin and its congener prodrugs in the horse

Ampicillin is an antibiotic commonly administered to horses by both the intramuscular (i.m.) and the intravenous (i.v.) route. Its physicochemical properties restrict its absorption after oral administration and explain its rapid elimination from the body. To prolong the effects of ampicillin in the horse, attempts have been made to alter its elimination and absorption rates. The alteration of urinary pH did not change the plasma disposition of the antibiotic but when probenecid was administered concurrently with ampicillin, a significant reduction of total body clearance was achieved. Ampicillin may also be maintained in the equine body, for a prolonged period of time when administered as an i.v. infusion. However, the practical difficulties associated with this route of administration and the limited potential advantage over conventional routes such as i.m. injection restrict its application to the critically ill animal and the perioperative period. When bacampicillin and pivampicillin (two ampicillin prodrugs) were administered to horses, high oral bioavailability was obtained, and the use of prodrugs commands the need for further investigation. The use of ampicillin might be limited in the future as an increase in the number of resistant equine bacterial strains emerge and it may be prudent to restrict its use according to the principles of good clinical pharmacological practice.

Reversed-phase high-performance liquid chromatography of amphoteric beta-lactam antibiotics: effects of columns, ion-pairing reagents and mobile phase pH on their retention times

The separation of five amino beta-lactam antibiotics by reversed-phase high-performance liquid chromatography was studied as an insight into their retention behaviour. These five amphoteric compounds are cephradine, cephalexine, cefaclor, ampicillin and amoxicillin. Both octadecylsilane-bonded silica (C18) columns and phenyl-bonded silica (phenyl) columns were used, with mobile phase pH values between 2.5 and 7.4. In the absence of ion-pairing reagents the retention times for all the five compounds were the shortest at pH 4-6. The phenyl column was found to improve the separation between cephradine and ampicillin at pH values lower than 3, when these two compounds appeared as fused peaks on the C18 on C18 columns, with mobile phases both with and without ion-pairing reagents, were compared. The addition of 0.005 or 0.02 M tetraethylammonium acetate to the mobile phase did not result in significant ion-pair formation, except at pH values higher than 5.5. A strong ion-pairing effect was obtained at pH values higher than 6 with 0.005 or 0.02 M tetrabutylammonium phosphate, and the retention was decreased at pH values lower than 4. On the other hand, 0.005 M heptanesulphonic acid exhibited an ion-pair retention effect at pH values lower than 5. The molecular structures and pK(a) values were used to account for the retention behaviour of these antibiotics in the various mobile phases.

Dissolution studies on ampicillin embonate and amoxycillin embonate

The dissolution behaviour of slightly water-soluble embonic acid salts of ampicillin and amoxycillin was studied quantitatively as a function of solution pH (1.15-8.00) using the rotating disk method combined with reversed-phase high-performance liquid chromatography. The dissolution rate of ampicillin embonate was greater than that of amoxycillin embonate at all pH values investigated. The graphs of pH-intrinsic rate of dissolution of the antibiotics from the salts were U-shaped, the minimum being close to the respective isoelectric points of the parent antibiotics. Embonic acid was not detectable below pH 5; above pH 5 the dissolution rate of embonic acid increased as a function of pH.

Potential improvement in the shelf life of parenterals using the prodrug approach: bacampicillin and talampicillin hydrolysis kinetics and utilization time

The utilization time for a parenteral prodrug solution with a bioavailable fraction of unity was defined as the time during which the total of the prodrug concentration and the drug concentration equals or exceeds 90% of the initial prodrug concentration. This utilization time was calculated as a function of pH, buffer, and temperature using the experimentally determined rate expressions for bacampicillin and talampicillin. The results were compared to the shelf life of ampicillin solutions under identical storage conditions. First-order rate constants were determined for conversion of the prodrugs to ampicillin (kc), for beta-lactam degradation of the prodrugs (knc), for the overall loss of prodrugs (ksum), and for beta-lactam degradation of ampicillin (kh) in aqueous solutions at 25.0 to 60.0 degrees C, mu = 0.5, in the pH range 0.90 to 8.4. Loss of bacampicillin proceeded primarily by degradation at pH levels below 4 but was due predominantly to conversion at pH levels above 5. Loss of talampicillin was due primarily to conversion throughout the entire pH range. While the prodrug utilization times were approximately twice the shelf life of ampicillin in acidic solutions, ampicillin was significantly better in neutral solutions. The results illustrate the potential for increased prodrug storage periods when utilization time is defined on the basis of the bioactivity rather than on the prodrug concentration alone.

The pH dependence of the active-site serine DD-peptidase of Streptomyces R61

Titration of the active-site serine DD-peptidase of Streptomyces R61 shows that formation of acyl enzyme during hydrolysis of the substrate Ac2-L-Lys-D-Ala-D-Ala and enzyme inactivation by the beta-lactam compounds benzylpenicillin, N-acetylampicillin and ampicillin relies on the acidic form of an enzyme's group of pK approximately equal to 9.5. It is proposed that protonation of a lysine epsilon-amino group facilitates initial binding by charge pairing with the free carboxylate of the substrate and the beta-lactam molecules. Lowering the pH from 7 to 5 has no effect on the second-order rate constant of enzyme acylation by benzylpenicillin and N-acetylampicillin but results in a decreased rate constant of acylation by ampicillin and Ac2-L-Lys-D-Ala-D-Ala. Protonation of the side-chain amino group of ampicillin and a decreased efficacy of the initial binding of the peptide to the enzyme seem to be responsible for the observed effects. Whatever the molecule bound to the enzyme, there is no sign for the active involvement of an enzyme's histidine residue of pK 6.5-7.0 in the hydrolysis pathway.

Physicochemical properties of amphoteric beta-lactam antibiotics. II: Solubility and dissolution behavior of aminocephalosporins as a function of pH

The solubility of aminocephalosporins in aqueous solution at 37 degrees and an ionic strength of 0.5 exhibited U-shaped curves against pH. At their isoelectric pH, cephradine monohydrate was the most soluble, followed by cephalexin monohydrate and cephaloglycin dihydrate, with intrinsic solubilities of 26.0, 17.2, and 14.8 mg/ml, respectively. The dissolution rate constants from the rotating disk were also determined as a function of the pH of the dissolution medium and interpreted reasonably by the simultaneous dissociation equilibrium reaction and the diffusion kinetics model. Energies for the solubility and dissolution were determined for these three aminocephalosporins.

Physicochemical properties of amphoteric beta-lactam antibiotics I: Stability, solubility, and dissolution behavior of amino penicillins as a function of pH

The degradation rate, solubility, and dissolution rate of amino penicillins, amoxicillin, ampicillin, epicillin, and cyclacillin, were determined quantitatively as a function of pH. In the pH range studied, 0.30-10.50, the degradation of amoxicillin and epicillin followed pseudo-first-order kinetics to give the same type of pH-rate profiles as those of ampicillin and cyclacillin. Cyclacillin anhydrate was the most soluble, followed in order by ampicillin anhydrate, ampicillin trihydrate, amoxicillin trihydrate, and epicillin anhydrate. These pH-solubility profiles showed showed U-shaped curves. The dissolution rate constants from the rotating disk were analyzed by the simultaneous chemical reaction and diffusion models. Their relative bioavailability after a single oral administration was assessed from their physicochemical properties determined in vitro.

Physicochemical properties of beta-lactam antibiotics: oil-water distribution

Apparent partition coefficients Papp, of beta-lactam antibiotics were determined in octanol-water and 2-methylpropanol-water systems at various pH values. The pKa values also were determined by potentiometry under the conditions of partition experiments. The intrinsic partition coefficients for the unionized form, Pu, and the ionized form, Pi, of beta-lactam antibiotics were calculated from the equation Papp = Pu[aH+/(Ka + aH+)] + Pi[Ka/(Ka + aH+)]. The correlation between Pu and Pi values and lipophilic parameters of penicillins measured in other systems was examined.

Absorption of amino penicillins from everted rat intestine

1. Using an in vitro everted gut sac method based on that of Wilson & Wiseman (1954), a number of amino penicillins were tested in order to identify the involvement of any specialized transport mechanisms in their absorption across rat intestine. 2. Only one of the amino penicillins, cyclacillin (1-amino-cyclohexyl penicillin) was shown to be actively transported. The other penicillins appeared to diffuse passively across the intestine. 3. Cyclacillin was found to concentrate against a gradient at 37 degrees C but not at 19 degrees C. 4. Transport of cyclacillin across the mucosal membrane was saturated at mucosal concentrations greater than 1000 microgram/ml. 5. The rate of the forward flux of cyclacillin was many times that of its back flux. 6. No relationship between the active transport of cyclacillin and that of amino acids could be demonstrated.

Rapid, precise, turbidometric assay for low levels of ampicillin in serum after single-dose oral administration

A turbidometric assay is described for the quantitative measurement of ampicillin in serum. Standard curves prepared with known concentrations of ampicillin in serum exhibited acceptable linearity over a concentration range of approximately 0.2 to 1.8 mug/ml. Data are presented to show the excellent precision of the assay and the application of the assay to clinical studies. The advantages of this method over other procedures are discussed. Because of the questionable stability of ampicillin, samples containing known concentrations of ampicillin in serum were assayed after storage for various lengths of time. Serum samples maintained in the frozen state until the time of assay exhibited approximately 12% degradation after 7 days, whereas those samples which were subjected to repeated thawing and refreezing exhibited approximately 25% degradation after the same time interval.